The invention relates to cutting at least one thin layer from a substrate or ingot, in particular of semiconductor material(s), for making an electronic, optoelectronic or optical component or sensor.
In numerous applications associated with the fields of microelectronics, optoelectronics, and sensors, the technological operation of transferring a layer of one substrate onto another represents a key operation that enables numerous materials structures or specific components to be fabricated. The layer for transfer may or may not include components that are complete or in a partial state of completion.
One example of such applications is in making silicon on insulator (SOI) substrates. Typically, the insulator used is SiO2 of amorphous structure on which it is not possible to deposit silicon of monocrystalline quality. One category of techniques for making such structures relies on molecular adhesion techniques referred to as “wafer bonding”. These techniques are known to the person skilled in the art and in particular are described in the text “Semiconductor Wafer Bonding Science and Technology” by Q. Y. Tong and U. Gösele, a Wiley Interscience Publication, Johnson Wiley & Sons. Inc. As described in that text, using such techniques, two substrates (generally silicon substrates) are assembled together, one that is used to form the SOI layer (the “source” substrate) that is to be transferred onto the other substrate. This other substrate thus becomes the new “support” substrate that supports the SOI layer. A layer of insulation, typically of SiO2 is previously formed on at least one of the faces of the substrates prior to assembly, thus obtaining a buried insulator situated beneath the SOI layer.
Certain variants are known as “bonded SOI” (BSOI) or indeed “bond etch back SOI” (BESOI). In addition to molecular adhesion, these variants rely on physically removing the source substrate either by techniques of the polishing or mechanical lapping type and/or by techniques of the chemical etching type. Other variants rely on splitting along a zone of weakness in addition to molecular adhesion on separation. These methods are described in U.S. Pat. No. 5,374,564 (or EP-A-0 533 551) and U.S. Pat. No. 6,020,252 (or EP-A-0 807 970) where splitting occurs along a weakened zone of implanted ions, or in European patent application 0 925 888, where splitting occurs through a buried layer that has been made porous.
Those layer transfer techniques present are of a generic nature since they enable structures to be made that combine various types of material with one another, and specifically that they enable structures to be obtained that are not possible to make otherwise, and in particular by deposition. Examples are monocrystalline silicon substrates on quartz, AsGa substrates on silicon, and the like.
The advantage of the methods that split along a buried fragile layer is that it is possible to make layers based on crystalline silicon (or on SiC, InP, AsGa, LinbO3, LiTaO3, and the like) in a range of thicknesses that extends from a few tens of angstroms (Å) to a few micrometers (μm), with very good uniformity. Even greater thicknesses are possible. other examples of applications in which layer transfer techniques can provide a suitable solution for integrating components or layers on a support that would otherwise be unsuitable for receiving such components or layers. These layer transfer techniques are also very useful when it is desired to isolate a fine layer, with or without components, from its initial substrate, e.g., by separating or eliminating the substrate.
By way of example, more and more components are expected to be integrated on supports that are different from those which enabled them to be made. By way of example, mention can in addition to making substrates, there are numerous be made of components on substrates made of plastics or on substrates that are flexible. The term “components” is used herein to mean any microelectronic device, optoelectronic device, or sensor device (e.g. a chemical, mechanical, thermal, biological, or biochemical sensor device) that is fully or partially “processed”, i.e. that has been made in full or in part. In order to integrate such components on flexible supports that are otherwise incompatible with such components, it is possible to use a layer transfer method which is performed after the components have been made on a substrate which is compatible with them.
Still in the same spirit, turning a fine layer over while transferring it to another support provides engineers with a degree of freedom that is very useful for designing structures that would otherwise be impossible. Taking and turning over such thin films make it possible, for example, to make so-called “buried” structures such as buried capacitors for dynamic random access memories (DRAMs) where, contrary to the usual case, the capacitors are made first and then transferred onto another silicon substrate, after which the remainder of the circuits are fabricated on the new substrate. Another example lies in the manufactures of double gate transistors. The first gate of a CMOS transistor is made using conventional technology on one substrate, and it is then turned over and transferred onto a second substrate where the second gate of the transistor is made and the transistor is finished, thus leaving the first gate buried within the structure (see for example K. Suzuki, T. Tanaka, Y. Tosaka, H. Horie, and T. Sugii, “High speed and low power n+-p+ double gate SOI CMOS”, IEICE Trans. Electron., Vol. E78-C, 1995, pp. 360-367).
An identical situation is to be found for example in the field of applications associated with telecommunications and microwaves. Under such circumstances, it is preferable for components finally to be integrated on a support presenting high resistivity, typically several kilo ohm-centimeters (kΩ.cm) at least. However a highly resistive substrate is not necessarily available at the same cost and quality as the standard substrates that are usually used. With silicon, silicon wafers having a diameter of 200 millimeters (mm) and wafers having a diameter of 300 mm are available at standard resistivity, whereas for resistivities greater than 1 kΩ.cm availability is quite inadequate at 200 mm and non-existent at 300 mm. One solution consists in making the components on standard substrates and then in transferring them during the final stages to a fine layer containing components on an insulating substrate of glass, quartz, sapphire, or the like.
From a technical point of view, these transfer operations have the major advantage of de-correlating the properties of the layer in which the components are made from those of the final support layer, and consequently they are advantageous in many other circumstances.
Relating more specifically to layer transfer techniques based on the splitting (i.e., breaking or separating) along a zone of weakness (“weakness” to be understood broadly and from a mechanical point of view) or a zone predefined to originate separation selectively (e.g., separation by chemical etching), several techniques are known concerning the step or combination that gives rise to the cut.
For example, certain combinations are based more specifically on mechanical separation (e.g., the high pressure water jet disclosed in EP 0 925 888). Certain techniques based on the so-called “lift-off” principle also enable a thin layer to be separated from the remainder of the initial support, without necessarily consuming it. Those methods generally make use of chemical etching that acts selectively on a buried intermediate layer, optionally associated with the application of mechanical forces. That type of method is in widespread use for transferring III-V elements on to various types of support (see C. Camperi et al. IEEE Transactions and Photonics Technology, Vol. 3, 12 (1991) 1123).
As another example, EP 0 925 888 describes slitting by means of a fracture along a buried layer that is made porous by mechanical means represented by a jet of water under pressure applied in the vicinity of the zone to be cut. A jet of compressed air can also be used as described in French patent application FR 2 796 491, or it is also possible to exert traction as disclosed in PCT published application WO 00/26000. It can also be appropriate to insert a blade.
Other examples rely on a zone of weakness obtained by implantation. A cut can be obtained along this zone of weakness, optionally by combining said implantation with the specific means for applying mechanical forces as mentioned above (or other such means) and/or chemical etching and/or heat treatments, etc. A few examples of such techniques are to be found in documents U.S. Pat. No. 5,374,564 (or EP-A-0 533 551) and U.S. Pat. No. 6,020,252 (or EP-A-0 807 970), and PCT published application WO 00/61841.
Numerous means can be adopted to trigger or assist splitting along a zone of weakness. U.S. Pat. Nos. 6,020,252 and 6,013,563 and European patent applications 0 961 312 and 1 014 452 provide more detailed explanations of, for example, mechanical forces in tension, in shear, in twisting, heat treatments using a wide variety of hot or cold sources of heat (conventional ovens, light means, lasers, electromagnetic fields, electron beams, cryogenic fluids, etc.), laser ablation of an intermediate layer, and the like.
The layer transfer techniques mentioned in the introduction nevertheless present certain specific drawbacks.
Techniques based on thinning down (mechanically, chemically, etc.) suffer from the drawback of consuming and sacrificing a substrate, which is inefficient from an economic standpoint. Such thinning techniques are also often quite difficult and expensive to implement.
Combinations based on applying external mechanical stresses (shear, twisting, bending, tension, and the like) suffer from the drawback of generally requiring adhesion (molecular or otherwise) that is sufficiently strong to avoid breaking under the stress needed for rupturing the weak zone. A method for obtaining such adhesion is not always available in certain manufacturing methods or applications which are subject to very severe specifications (e.g., where it is impossible to heat, impossible to use specific solvents or other chemicals, impossible to apply traction to the structure because of the risk of destroying sensitive components, etc.).
In certain applications, techniques based on annealing and other heat treatments come up against incompatibility with the step of raising temperature, e.g., the temperature of the final support on which the layer is to be integrated. For example, the new support may not be capable of withstanding the temperatures required. This generally applies to plastic materials. By way of another example, the incompatibility can stem from the combination of materials, in particular because they have too great a difference in thermal expansion coefficients which would cause an assembly that is not sufficiently uniform to break during a temperature rise. This would apply for example to a structure that combines silicon and quartz.
Techniques based on chemical etching are aggressive and this can make them incompatible with the final support on which the layer for transfer is to be integrated, or with components that might already be present on that layer.
Among other combinations, U.S. Pat. No. 6,013,563 and European patent application 1 014 452 describe or mention techniques based on applying beams of light and/or electrons. U.S. Pat. No. 6,013,563 refers to applying a beam of photons and/or electrons in order to heat the structure, while EP 1 014 452 describes a method in which an arbitrary source of photons (X rays, UV light, visible light, infrared light, microwaves, lasers, etc.) is suitable for giving rise to separation. The implementation described when using a laser, for example, refers to laser ablation of the intermediate layer which leads the authors to prefer using laser pulses of relatively high power (“preferably for energy densities lying in the range 100 millijoules per square centimeter (mJ/cm3) and 500 mj/cm3”) and of relatively long duration (“preferably for durations lying in the range 1 nanosecond (ns) to 1000 ns, and especially for durations lying in the range 10 ns to 100 ns”). The authors also state that that method of implementation requiring relatively large amounts of energy to be delivered in order to operate suffers from the drawback of possibly damaging the layer that is to be transferred.
Thus there is a need for further manufacturing processes that do not possess the disadvantages of the prior art.